Seawater desalination installations using reverse osmosis processes have increased significantly within the past five years. Prior to 1973, the primary systems for seawater conversion by reverse osmosis was the two-stage, two-pump unit. Separate types of modified cellulose acetate membrane modules were employed in two separate reduction stages to reach a potable product below 500 PPM total dissolved solids. With the introduction of a new process membrane by DuPont and others, the hollow fine fiber module, designs employed only one stage using one pump at a higher operative pressure to accomplish desalination. In the two stage systems, the classical pre-treatment of the membrane feed consisted of filtration, injection of chlorine and acid and polyphosphate. This enabled the sanitizing and pH adjustment and a scale prevention for maintaining membrane life. The hollow fine fiber type employing a polyamide polymer material can not tolerate free chlorine: pre-treatment and pre-filtration and chemical injection have become more critical. The post treatment in both cases employed decarbination and often post-neutralization and chlorination for potable storage.
The above systems are designed for a large demand installation such as commercial and municipal installations, which are primarily for land based areas. Very little work is currently being undertaken for the small privately maintained "point of use" areas. These areas specifically include the marine (shipboard), land-sea based (offshore), and the remote installations of the world. These areas have limited low power source available and lack sources of chemical for pre and post treatment methods. Therefore, this invention was conceived in order to fulfill the need for a small, compact, light-weight, seawater desalination plant for marine and remote land-sea based applications.
Unit "A", shown in FIGS. 1-2, employs one pump stage to feed a membrane module consisting of a modified cellulose tri-acetate. This module is capable of reducing the seawater feed salinity by 80% total dissolved solids. The product from this module is stored for a short period of time and recycled through the pump by valve means to a second and separate membrane module consisting of a modified cellulose acetate. This module is capable of reducing the feed salinity by 95% or more. This module is capable of higher rejection to salts but cannot withstand the higher seawater feed salinity that is fed through the first module. The concentrate from the second module is diverted back under pressure, through the first membrane module. This stream functions to rinse or flush the membrane of scale forming compounds that tend to collect on the membrane boundary layers. This concentrate is approximately one-third of the seawater salinity. The rinsing and diluting is required for maintaining longer membrane life.
This unit, unlike Unit B, utilizes two separate membranes in two steps of recycle storage functions to accomplish desalination of the feed water. However, it produces a greater volume of potable product for the same power used in Unit "B". This invention does not use acid or poly phosphate injection for scale prevention. Therefore, post treatment is not required. The unit does employ a small chemical feeder which periodically can inject a chlorine solution for proper sanitization of the unit.
Unit "B", shown in FIGS. 4-6, employs one pump stage to feed a membrane of modified cellulose triacetate. The permeate is stored for a short period of time and by valve means, is re-cycled. The product (permeate) from the second step is stored again and recycled for a third pass. The primary function of the recycle concept is to reduce the salinity of the feed in smaller rejection steps. The secondary function of the re-cycle concept is to flush or rinse the membrane surfaces or (boundary layers) with successively purer water. This reduces scale forming tendencies and maintains a longer membrane life. The one pump stage employed in this invention uses lower power requirements than conventional two-pump, two-stage systems. This unit employs lower pressures than newer designed hollow fine fiber systems, thus reducing the compaction of the membrane and corresponding loss of productivity over a period of time. By eliminating acid injection and scale inhibitor chemicals, the post treatment or post neutralization of the product is eliminated. This invention also employs light-weight plastic components for weight and size reduction. A further version, Unit C, later will be described.
A search was made resulting in citation of the following numbered patents by the searcher:
U.S. Pat. No. 3,498,072 PA1 U.S. Pat. No. 3,589,998 PA1 U.S. Pat. No. 3,617,550 PA1 U.S. Pat. No. 3,786,924 PA1 U.S. Pat. No. 3,821,108 PA1 U.S. Pat. No. 3,836,457 PA1 U.S. Pat. No. 3,846,295 PA1 U.S. Pat. No. 3,898,158 PA1 U.S. Pat. No. 3,956,114 PA1 U.S. Pat. No. 4,000,065 PA1 U.S. Pat. No. 4,005,012 PA1 U.S. Pat. No. 4,071,445 PA1 U.S. Pat. No. 4,080,289 PA1 U.S. Pat. No. 4,083,781
I was not familiar with the identical systems and equipment described in these patents. I don't consider most of the patents to be at all pertinent to the system described herein. U.S. Pat. No. 4,083,781 does recycle product water. U.S. Pat. Nos. 4,000,065 and 3,589,998 recycle concentrated solutions. The objective in U.S. Pat. No. 3,617,550 is to concentrate rather than to purify, but the "purer" fraction passing through a membrane 16 of the patent does pass to a feed tank 10 and then back through a membrane 14. These recycling concepts are of limited pertinency and none of the patents, individually or collectively, teach the concepts described as new herein.